Late lean injection for fuel flexibility

ABSTRACT

A gas turbine engine is provided and includes a combustor having a first interior in which a first fuel is combustible, the first fuel including natural gas and/or a blend of natural gas and alternate gas receivable by a fuel circuit from an external source, a turbine, a transition zone, including a second interior in which a second fuel, the second fuel including an unblended supply of the alternate gas receivable by the fuel circuit from the external source, and the products of the combustion of the first fuel are combustible, and a plurality of fuel injectors, which are structurally supported by the transition zone and coupled to the fuel circuit, and which are configured to supply the second fuel to the second interior in any one of a single axial stage, multiple axial stages, a single axial circumferential stage and multiple axial circumferential stages.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the following applications: “Late LeanInjection with Expanded Fuel Flexibility”; “Late Lean Injection SystemConfiguration”; “Late Lean Injection Fuel Injector Configurations”;“Late Lean Injection with Adjustable Air Splits”; “Late Lean InjectionFuel Staging Configurations” and “Late Lean Injection Control Strategy”,each of which is being filed concurrently herewith and the contents ofwhich are incorporated herein by reference thereto.

BACKGROUND OF THE INVENTION

Aspects of the present invention are directed to late lean injection(LLI) fuel staging configurations and methods of achieving the same.

Currently, some gas turbine engines fail to operate at high efficienciesand produce undesirable air polluting emissions. The primary airpolluting emissions usually produced by turbines burning conventionalhydrocarbon fuels are oxides of nitrogen, carbon monoxide and unburnedhydrocarbons. To this end, since oxidation of, e.g., molecular nitrogen,in gas turbine engines is dependent upon a high temperature in thecombustor and the residence time for the reactants at the hightemperature within the combustor, a level of thermal NOx formation isreduced by maintaining the combustor temperature below the level atwhich thermal NOx is formed or by limiting the residence time for thereactants at the high temperatures such that there is insufficient timefor the NOx formation reactions to progress.

One temperature controlling method involves the premixing of fuel andair to form a lean mixture thereof prior to combustion. However, it hasbeen seen that, for heavy duty industrial gas turbines, even with theuse of premixed lean fuels, the required temperatures of the combustionproducts are so high that the combustor must be operated with peak gastemperatures in the reaction zone that exceed the thermal NOx formationthreshold temperature, resulting in significant NOx formation.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a gas turbine engine isprovided and includes a combustor having a first interior in which afirst fuel supplied thereto by a fuel circuit is combustible, the firstfuel including natural gas and/or a blend of natural gas and alternategas receivable by the fuel circuit from an external source, a turbine,including rotating turbine blades, into which products of at least thecombustion of the first fuel are receivable to power the rotation of theturbine blades, a transition zone, including a second interior in whicha second fuel supplied thereto by the fuel circuit, the second fuelincluding an unblended supply of the alternate gas receivable by thefuel circuit from the external source, and the products of thecombustion of the first fuel are combustible, the transition zone beingdisposed to fluidly couple the combustor and the turbine to one another,and a plurality of fuel injectors, which are structurally supported bythe transition zone and coupled to the fuel circuit, and which areconfigured to supply the second fuel to the second interior in any oneof a single axial stage, multiple axial stages, a single axialcircumferential stage and multiple axial circumferential stages.

According to another aspect of the invention, a gas turbine engineincluding a combustor having a first interior in which a first fuelsupplied thereto by a fuel circuit is combustible, the first fuelincluding natural gas and/or a blend of natural gas and alternate gasreceivable by the fuel circuit from an external source, and a turbine,including rotating turbine blades, into which products of at least thecombustion of the first fuel are receivable to power the rotation of theturbine blades, is provided and includes a transition zone, including asecond interior in which a second fuel supplied thereto by the fuelcircuit, the second fuel including an unblended supply of the alternategas receivable by the fuel circuit from the external source, and theproducts of the combustion of the first fuel are combustible, thetransition zone being disposed to fluidly couple the combustor and theturbine to one another, and a plurality of fuel injectors, which arestructurally supported by the transition zone and coupled to the fuelcircuit, and which are configured to supply the second fuel to thesecond interior in any one of a single axial stage, multiple axialstages, a single axial circumferential stage and multiple axialcircumferential stages.

According to yet another aspect of the invention, a method of operatinga gas turbine engine in which a turbine is fluidly coupled to acombustor by a transition zone interposed therebetween, is provided andincludes supplying a first fuel to a first interior within thecombustor, the first fuel including natural gas and/or a blend of thenatural gas and alternate gas, combusting the first fuel in the firstinterior within the combustor, supplying a second fuel, including anunblended supply of the alternate gas, to a second interior within thetransition zone in any one of a single axial stage, multiple axialstages, a single axial circumferential stage and multiple axialcircumferential stages, and combusting the second fuel and a stream ofcombustion products, received from the first interior, in the secondinterior within the transition zone.

These and other advantages and features will become more apparent fromthe following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 is a side sectional view of a turbine including late leaninjection capability;

FIG. 2 is a graph illustrating a plot of a head end fuel splitpercentage versus a firing temperature of the turbine of FIG. 1;

FIG. 3 is a flow diagram illustrating a method of operating the turbineof FIG. 1;

FIGS. 4A-4D are side sectional views of various head end configurations;and

FIGS. 5A-5D are perspective views of various fuel injectorconfigurations.

The detailed description explains embodiments of the invention, togetherwith advantages and features, by way of example with reference to thedrawings.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, a gas turbine engine 10 is provided andincludes a combustor 20 having a first interior 21 in which a first fuelsupplied thereto by fuel circuit 70 is combustible, a compressor 30 bywhich inlet air is compressed and provided to at least the combustor 20and a transition zone 43 and a turbine 50, including rotating turbineblades, into which products of at least the combustion of the first fuelare receivable to power a rotation of the turbine blades. The transitionzone 43 is disposed to fluidly couple the combustor 20 and the turbine50 and includes a second interior 41 in which a second fuel suppliedthereto by the fuel circuit 70 and the products of the combustion of thefirst fuel are combustible. As shown, the combustor 20 and thetransition zone 43 combine with one another to generally have a form ofa head end 11, which may have various configurations, as will bediscussed below.

As shown in FIG. 1, the head end 11 may include multiple premixingnozzles 12. However, as shown in FIGS. 4A-4D, other head end 11configurations are possible. Such alternate configurations include, butare not limited to, the standard combustor configuration 13 of FIG. 4A,the Dry Low NOx (DLN) 1+ combustor configuration 14 of FIG. 4B, the DLN2+ combustor configuration 15 of FIG. 4C and the DLN 2.6/2.6+ combustorconfiguration 16 of FIG. 4D. Still other combustor configurationsinclude Integrated Gasification Combined Cycle (IGCC) head ends,catalytic head ends, diffusion style head ends and Multi-Nozzle QuietCombustion (MNQC) style head ends.

For each of the above-noted head end 11 configurations, it is understoodthat versions of the configurations may be late lean injection (LLI)compatible. An LLI compatible combustor is any combustor with either anexit temperature that exceeds 2500° F. or handles fuels with componentsthat are more reactive than methane with a hot side residence timegreater than 10 ms. As an example, an LLI compatible version of the DLN1+ combustor configuration 14 may have an exit temperature that is lessthan 2500° F. but may handle fuels with components that are morereactive than methane. An LLI compatible version of a diffusion stylehead end combustor may have exit temperatures in excess of 2500° F. andmay handle fuels with components that are more reactive than methane.Similarly, an LLI compatible version of the DLN 2.0/DLN 2+ combustorconfiguration 15 may have an exit temperature in excess of 2500° F. andmay handle fuels with components that are more reactive than methanewhile an LLI compatible version of catalytic head ends or the DLN2.6/2.6+ combustor configuration 16 may have an exit temperature inexcess of 2500° F. and may handle fuels with components more reactivethan methane.

A plurality of fuel injectors 60 are each structurally supported by anexterior wall of the transition zone 43 or by an exterior wall of asleeve 40 around the transition zone 43 and extend into the secondinterior 41 to varying depths. With this configuration, the fuelinjectors 60 are each configured to provide LLI fuel staging capability.That is, the fuel injectors 60 are each configured to supply the secondfuel (i.e., LLI fuel) to the second interior 41 by, e.g., fuel injectionin a direction that is generally transverse to a predominant flowdirection through the transition zone 43, in any one of a single axialstage, multiple axial stages, a single axial circumferential stage andmultiple axial circumferential stages. In so doing, conditions withinthe combustor 20 and the transition zone 43 are staged to create localzones of stable combustion.

With reference to FIGS. 5A-5D, it is seen that the fuel injectors 60 canhave various features and functionalities. For example, as shown in FIG.5A, the fuel injectors 60 can include a tube-in-tube injectorconfiguration 125. In this configuration, fuel is actively fed to theinterior 41 of the transition zone 43 through a nozzle in a tube 130 andair is passively fed through an annular space between the tube 130 andthe sleeve 145 which extends from the impingement sleeve surface 140 tothe interface part 150. As shown in FIG. 5B, the fuel injectors 60 canalso include swirl injectors 155. In this configuration, fuel isactively fed to the interior 41 of the transition zone 43 through amanifold 160 and air is passively fed through a central purge 165 and/orby way of a swirler 170. In addition, as shown in FIGS. 5C and 5D, otherexamples of fuel injector 60 configurations include a rich catalyticinjector configuration 175, which includes rich catalytic elements 180,and multi-tube/showerhead injector configurations 185, which includemultiple tubes 190 through which fuel is fed to the transition zone 43.In each of these cases, it is understood that the fuel injectors 60 canbe coupled to the transition zone 43 at locations that correspond topre-existing dilution holes 42, if any, on the outer surface of thetransition zone 43. In this way, since additional holes need not bedrilled into the outer surface of the transition zone 43, manufacturingcosts and negative performance effects are limited or substantiallyreduced. If dilution holes 42 do not exist already, fuel injectors 60may be placed as required on the exterior of the transition zone 43.

In accordance with embodiments, the single axial stage includes acurrently operating single fuel injector 60, the multiple axial stagesthere is no antecedent basis for multiple here include multiplecurrently operating fuel injectors 60, which are respectively disposedat multiple axial locations of the transition zone 43, the single axialcircumferential stage includes multiple currently operating fuelinjectors 60 respectively disposed around a circumference of a singleaxial location of the transition zone 43, and the multiple axialcircumferential stages include multiple currently operating fuelinjectors 60, which are disposed around a circumference of thetransition zone 43 at multiple axial locations thereof.

Here, where multiple fuel injectors 60 are disposed around acircumference of the transition zone 43, the fuel injectors 60 may bespaced substantially evenly or unevenly from one another. As an example,eight or ten fuel injectors 60 may be employed at a particularcircumferential stage with 2, 3, 4 or 5 fuel injectors 60 installed withvarying degrees of separation from one another on northern and southernhemispheres of the transition zone 43. Also, where multiple fuelinjectors 60 are disposed at multiple axial stages of the transitionzone 43, the fuel injectors 60 may be in-line and/or staggered withrespect to one another.

During operations of the gas turbine engine 10, each of the fuelinjectors 60 may be jointly or separately activated or deactivated so asto form the currently effective one of the single axial stage, themultiple axial stages, the single axial circumferential stage and themultiple axial circumferential stages. To this end, it is understoodthat the fuel injectors 60 may each be supplied with LLI fuel by way ofthe fuel circuit 70 via a valve 61 disposed between a corresponding fuelinjector 60 and a branch 71 or 72 of the fuel circuit 70. The valve 61signal communicates with a controller 80 that sends a signal to thevalve 61 that causes the valve 61 to open or close and to therebyactivate or deactivate the corresponding fuel injector 60.

Thus, if it is currently desirable to have each fuel injector 60currently activated (i.e., multiple axial circumferential stages), thecontroller 80 signals to each of the valves 61 to open and therebyactivate each of the fuel injectors 60. Conversely, if it is currentlydesirable to have each fuel injector 60 of a particular axial stage ofthe transition zone 43 currently activated (i.e., single axialcircumferential stage), the controller 80 signals to each of the valves61 corresponding to only the fuel injectors 60 of the single axialcircumferential stage to open and thereby activate each of the fuelinjectors 60. Of course, this control system is merely exemplary and itis understood that multiple combinations of fuel injector configurationsare possible and that other systems and methods for controlling at leastone of the activation and deactivation of the fuel injectors 60 areavailable.

In addition, with the presence of multiple fuel injectors 60 havingmultiple fuel injector configurations, as described above, thecontroller 80 may be further configured to activate only those fuelinjectors 60 having certain fuel injector configurations at any onetime. Thus, if it is currently desirable to have each fuel injector 60with a tube in tube injector configuration 125 currently activated, thecontroller 80 signals to each of the valves 61 corresponding to thosefuel injectors 60 to open. Conversely, if it is currently desirable tohave each fuel injector 60 that includes a swirl injector 155, thecontroller 80 signals to each of the valves 61 corresponding to only thefuel injectors 60 including swirl injectors 155 to open.

In accordance with another aspect of the invention, a method ofoperating a gas turbine engine 10, in which a turbine 50 is fluidlycoupled to a combustor 20 by a transition zone 43 interposedtherebetween, is provided. The method includes supplying a first fuel toa first interior 21 within the combustor 20, combusting the first fuelin the first interior 21 within the combustor 20, supplying a secondfuel to a second interior 41 within the transition zone 43 in any one ofa single axial stage, multiple axial stages, a single axialcircumferential stage and multiple axial circumferential stages, andcombusting the second fuel and a stream of combustion products, receivedfrom the first interior 21, in the second interior 41 within thetransition zone.

Here, the supplying of the second fuel to the second interior 41 in thesingle axial stage includes activating a single fuel injector 60, thesupplying of the second fuel to the second interior 41 in the multipleaxial stages includes activating multiple fuel injectors 60 respectivelydisposed at multiple axial locations of the transition zone 43, thesupplying of the second fuel to the second interior 41 in the singleaxial circumferential stage includes activating multiple fuel injectors60 respectively disposed around a circumference of the transition zone43 at a single axial location thereof, and the supplying of the secondfuel to the second interior 41 in the multiple axial circumferentialstages includes activating multiple fuel injectors 60 respectivelydisposed around a circumference of the transition zone 43 at multipleaxial locations thereof.

With reference to FIG. 2, it is seen that FIG. 2 provides a graphicalillustration of various options for fuel split controls. In FIG. 2, apercentage of a head end fuel split is plotted against a Tfire value,which is a measurement of a temperature within the combustor 20 and/orthe transition zone 43. Since the controller 80 is further configured tocontrol a flow direction of the first and second fuels toward the headend 11 and/or the fuel injectors 60, it is seen that LLI fuel stagingcan also be further controlled by the controller 80 in accordance withthe control options shown in FIG. 2. To this end, the controller 80 iscoupled to the valves 61, as discussed above, and the valve 73 tocontrol the delivery of the second fuel to the fuel injectors 60 and tocontrol the delivery of the first and/or the LLI fuel to the head end11.

As shown in FIG. 2, a first option for such control is to employ a fixedhead end split in which a percentage (<100%) of the first and/or the LLIfuel is delivered to the transition zone 43 for LLI fuel staging once apreselected value of Tfire is achieved. From that point, the percentageof the fuel delivered to the transition zone 43 remains substantiallyconstant. A second option is to employ a head end split schedule. Inthis case, the percentage of LLI fuel increases proportionally, or inaccordance with some other suitable function, as the value of Tfireincreases beyond a preselected value. A third option is to make thepercentage of the LLI fuel a function of any one or more of severalcharacteristics of the interior environments of the combustor 20, thecompressor 30, the transition zone 43 and/or the turbine 50. Forexample, the percentage could be made a function of Tcd or Pcd, whichare measured compressor discharge temperature and pressure conditions,humidity readings from within the combustor 20, the compressor 30 or thetransition zone 43, gas turbine exhaust temperature and/or T39, which isa calculated combustor exit temperature.

The third option discussed above may also be modified to account for acurrent Modified Wobbe Index (MWI). In this case, since it is known thatthe MWI measures energy density of fuel and that, for a given fuelnozzle area, a lower MWI indicates increased pressure ratios across thehead end fuel nozzles that can cause undesirable dynamics to which LLIfuel nozzles are insensitive, the percentage of the LLI fuel could alsobe made a function of the MWI. This way, as the MWI increases, a largerpercentage of LLI fuel is diverted to the transition zone 43. Inaccordance with each of these options, it is understood thatthermocouples/pressure gauges 100, or any other suitable environmentalmeasurement device, may be installed within the combustor 20, thecompressor 30, the transition zone 43 and/or the turbine 50 as is deemednecessary to measure temperatures and pressures within the combustor 20,the compressor 30, the transition zone 43 and the turbine 50.

With reference to FIG. 3, it is seen that a method of controlling aturbine with LLI capability includes operating the turbine 300,initiating the LLI 310 after a certain period of time or once apreselected value of Tfire is achieved, and, in accordance with thefirst option, continuing to operate the LLI at the same level 350.Conversely, in accordance with the second option, the method includescontinuing to operate the LLI at an increasing level 350. Meanwhile, inaccordance with the third option, it is determined whether anyparticular measured characteristics of the combustor 20, the compressor30 and/or the transition zone 43 are elevated or lower than establishedparameters 320 and, based on a result of the determining, the LLI levelis decreased 330, increased 340 or maintained and, subsequently, the LLIoperation is continued 350.

Still referring to FIG. 1, the controller 80 is further configured tocontrol the 3-way valve 110 and, in some embodiments, an additionalvalve disposed on a manifold around the fuel injectors 60 or, asmentioned above, the valves 61. Thus, the controller 80 is able tocontrol the air split of the inlet air delivered by the compressor 30 tothe combustor 20 and the transition zone 43 or to each fuel injector 60.In this way, the controller 80 is able to modify fuel splits and airsplits simultaneously. As such, the controller 80 can thereby createoperational paths for a combustion system that respect optimal fuel toair ratios of the combustion system. In accordance with variousembodiments of the invention, the 3-way valve 110 could be furtherintegrated as a part of an overall air coolant system, extended turndownefforts and/or Department of Energy (DoE) programs.

As described above, the control of the 3-way valve 110 is accomplishedin order to optimize fuel to air ratios of the combustion system. Theseratios may be preselected as being based on specifications for thecombustor 20 and the transition zone 43 or may be based on currentenvironmental conditions. In this case, the controller 80 could increasethe fuel to air ratio in either the combustor 20 or the transition zone43 based on temperature and/or pressure readings generated by thethermocouples/pressure gauges 100 installed within the combustor 20, thecompressor 30, the transition zone 43 and the turbine 50.

Late Lean Injection (LLI) can also allow for an injection of multiplegas streams, including alternate gases, such as refinery gases, into thetransition zone 43 that non-LLI combustors are generally unable tohandle. Highly reactive gases, such as refinery gases, typically cannotbe handled by premixed combustors due to the concern for undesirableflameholding in the premixers. Refinery gases on the other hand, whichmay or may not be blended with natural gases can, in certain cases, beinjected directly into the transition zone 43 without such problems,especially where the fuel injectors 60 are tolerant of flameholding.Here, where the refinery gases are blended with the natural gases, theamounts of the natural gases used can be a function of Tcd, Pcd, andT39, as described above. Also, where the refinery gases are injectedinto the transition zone 43, it is understood that flameholdingsensitive premixers can be employed at the head end 11 to prevent orsubstantially reduce the likelihood of flameholding incidents.

As shown in FIG. 1, the alternate gases can be injected from source 90into a branch 71 or 72 of the fuel circuit 70 via a refinery gas valve91 that is controlled by controller 80. This way, when it is determinedthat alternate gases are to be injected into the transition zone 43, thecontroller 80 can open the refinery gas valve 91 such that the alternategases can propagate through the fuel circuit 70 toward the fuelinjectors 61.

As a further embodiment, it is understood that the alternate gases canbe blended with natural gases to form the first fuel in compositionsthat reflect tolerances of the particular head end 11 in use. Thealternate gases can be provided with or without such blending to formthe second fuel.

In addition, it is further understood that the alternate gases mayinclude refinery gases that are received by the fuel circuit 70 from thesource 90, as mentioned above, and gases consisting of components thatare more reactive than methane. More particularly, the alternate gasesmay include gases that contain a quantity of above about 0.5% by volumeof hydrogen, a quantity of above about 5% by volume of ethane, aquantity of above about 10% by volume of propane, a quantity of aboveabout 5% by volume of butane or a hydrocarbon above butane.

The fuel circuit 70 may also incorporate multiple branches 71 and 72 toaccommodate for changes in fuel flow. The multiple branches 71 and 72can then also be used to allow for large changes in fuel composition byaffording additional fuel flow area or by introducing the fuel in a waythat creates separate modes of combustion (i.e. diffusion andpremixing). The branches 71 and 72 can also allow for variations in fuelwobbe number, fuel composition and for dynamic tuning. The branches 71and 72 of the fuel circuit 70 can be embodied as braches of the fuelcircuit 70, as additional fuel nozzles in the transition zone 43 or acombination of these options as well as other suitable options.

The branches 71 and 72 may further include a catalytic partial oxidationreactor (CPCR) 120 disposed along lengths thereof. The CPCR 120 convertsmethane within the first or second fuels to hydrogen and/or partiallyoxidizes the methane without creating nitrogen oxides. As a result,since the reacted fuel used for the LLI is already partially oxidized,the fuel can be injected into the transition zone 43 even later than itotherwise would be.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

1. A gas turbine engine comprising: a combustor having a first interiorin which a first fuel supplied thereto by a fuel circuit is combustible,the first fuel including natural gas and/or a blend of natural gas andalternate gas receivable by the fuel circuit from an external source; aturbine, including rotating turbine blades, into which products of atleast the combustion of the first fuel are receivable to power therotation of the turbine blades; a transition zone, including a secondinterior in which a second fuel supplied thereto by the fuel circuit,the second fuel including an unblended supply of the alternate gasreceivable by the fuel circuit from the external source, and theproducts of the combustion of the first fuel are combustible, thetransition zone being disposed to fluidly couple the combustor and theturbine to one another; and a plurality of fuel injectors, which arestructurally supported by the transition zone and coupled to the fuelcircuit, and which are configured to supply the second fuel to thesecond interior in any one of a single axial stage, multiple axialstages, a single axial circumferential stage and multiple axialcircumferential stages.
 2. The gas turbine engine according to claim 1,wherein the alternate gas comprises a refinery gas receivable by thefuel circuit from a gas supply.
 3. The gas turbine engine according toclaim 2, further comprising a control system configured to control anamount of the alternate gas receivable by the fuel circuit.
 4. The gasturbine engine according to claim 3, wherein the control systemcomprises: a valve disposed between and fluidly coupled to the fuelcircuit and the alternate gas supply; and a controller coupled to thevalve and configured to open and close the valve for at least one ofincreasing and decreasing the amount of the alternate gas receivable bythe fuel circuit.
 5. The gas turbine engine according to claim 4,wherein the controller at least one of increases and decreases theamount of the alternate gas in accordance with measured environmentalconditions.
 6. The gas turbine engine according to claim 1, wherein thealternate gas comprises components more reactive than methane.
 7. Thegas turbine according to claim 1, wherein the alternate gas comprises aquantity of above about 0.5% by volume hydrogen, a quantity of aboveabout 5% by volume ethane, a quantity of above about 0.5% by volumepropane, a quantity of above about 0.5% by volume butane and/or ahydrocarbon above butane.
 8. A gas turbine engine including a combustorhaving a first interior in which a first fuel supplied thereto by a fuelcircuit is combustible, the first fuel including natural gas and/or ablend of natural gas and alternate gas receivable by the fuel circuitfrom an external source, and a turbine, including rotating turbineblades, into which products of at least the combustion of the first fuelare receivable to power the rotation of the turbine blades, the gasturbine engine comprising: a transition zone, including a secondinterior in which a second fuel supplied thereto by the fuel circuit,the second fuel including an unblended supply of the alternate gasreceivable by the fuel circuit from the external source, and theproducts of the combustion of the first fuel are combustible, thetransition zone being disposed to fluidly couple the combustor and theturbine to one another; and a plurality of fuel injectors, which arestructurally supported by the transition zone and coupled to the fuelcircuit, and which are configured to supply the second fuel to thesecond interior in any one of a single axial stage, multiple axialstages, a single axial circumferential stage and multiple axialcircumferential stages.
 9. A method of operating a gas turbine engine inwhich a turbine is fluidly coupled to a combustor by a transition zoneinterposed therebetween, the method comprising: supplying a first fuelto a first interior within the combustor, the first fuel includingnatural gas and/or a blend of the natural gas and alternate gas;combusting the first fuel in the first interior within the combustor;supplying a second fuel, including an unblended supply of the alternategas, to a second interior within the transition zone in any one of asingle axial stage, multiple axial stages, a single axialcircumferential stage and multiple axial circumferential stages; andcombusting the second fuel and a stream of combustion products, receivedfrom the first interior, in the second interior within the transitionzone.
 10. The method according to claim 9, further comprising receivingthe natural gas from a natural gas source and the alternate gas from analternate gas source.
 11. The method according to claim 10, wherein thereceiving comprises controlling an amount of the received natural andalternate gas.
 12. The method according to claim 11, wherein thecontrolling of the amount of the received gas is achieved in accordancewith measured environmental conditions.